Yellow leaves. Can anyone help me say what I am doing wrong ..

Someone who may know what I'm doing wrong?
Have flowered for 3 weeks, never had a problem with yellow leaves, until when I switched to a stronger lamp.
very many leaves have fallen off in a short time. It's 10 days between the first two pictures so it goes fast.

Not given them any nutrition until now. Thought it was magnesium / Cal deficiency. Gave it a little over a week ago but doesn't get any better.
Let it dry out between I water it.

Could it be something with the roots? Have never had this problem before as I said but usually grow in much larger pots.
I have tried to read up on what I am doing wrong,
but just get confused because there are so many different errors you can get from yellow leaves.
Also have one more plant that starts to show the same symptoms as the first.

Uses a Telos 0006 180w. 18 hours of light per day. 50 cm distance from the top of the plant. 27degrees 40-50% humidity.
Help please!

Yellowing during flowering is normal. However, excessive heat has caused my plant's leaves to curl/dry up and then die. What's your NPK ratio on your food? Most likely the plants are requiring more food if you haven't fed them up until this point. Start off slowly with the feed, going below the recommended feeding strength is definitely a good start. Once your plants dry out, water-feed-water-feed and then so on. The ratio of food you use is important, but WHEN to use the food is more important. Generally, the leaves start yellow around week 4, but once I started using Jack's bloom booster, the plants have NPK and other micronutrients needed for proper growth, and they flourished. I feed all purpose Jack's classic 20-20-20 at 1 tsp per gal throughout veg and the first two weeks of flower. The plants are nice and dark green due to the higher nitrogen, but flowering requires less and less nitrogen as it progresses. Around week 3 in flower, pistils show and I switch to bloom booster 1 tsp per gal every other watering. If the temps are higher, the plants can suffer from nutrient lockout/burn. 75f with lights on during flower, 40%-50% RH. 65f-70f lights off during flower, 30%-40% RH and the nutrients will be used properly.